TELEGRAPH

The Electric, or, as it ought more properly to be called, the Electromagnetic Telegraph, has now superseded all otlier means of telegraphic communication. We purpose, therefore, here briefly to describe the principle upon which these instruments are constructed, referring for minute details to the various works which have been published, and to the specific at ons of the many patents which have been i ued upon the subject. The mutual relation of electric and magnetic currents will be evident from what has been stated elsewhere under the heads Electromagnetism and GalvaNOMETEK, so that it will only be nece ary to recapitulate such circumstances as have especial relation to telegraphic purposes. When a current of voltaic electricity is traversing'^a metallic wire, a magnetic current is at the same time e.'^tablished, at right angles to, and as it were revolving about the electric current, as its axis; and if the pole of a magnet be supposed capable of freely moving in any direction, the tendency of such pole would be to revolve about the wire in question, or rather, about the electric current which that wire carries. If a common magnetic needle, turning horizontally upon a point or pivot, be brought near the voltaic current, or to the wire conveying that current, the magnet will accordingly be deflected from its meridian. Supposing, for instance, the magnetic needle in its natural position, and pointing therefore north and south, to be approached by a wire transmitting the volta-electric current, held above, and parallel to the needle, the deflection of the nuedle will take place either to the right or left, or to the north or south, the direction of the deflection depending upon that of the electric current; and accordingly the tendency of the magnetic needle will be to place itself at right angles to the wire conveying the electric current. It is obvious, therefore, that the electric and magnetic forces may so far be said to deflect each other, and upon this principle the various forms of the galvanometer are 1409 TELEGRAPH. constructed. Now, in reference to the application of such principle to telegraphic purposes, another form of apparatus must be adopted, in which the magnetic needle and deflecting current, instead of being placed horizontally, as in the galvanometer, are placed vertically, or perpei.dicularly, as in the annexed diagram, where A B represents a wire having a magnetised steel needle placed immediately behind it, and so adjusted as to hang verticallv when at rest. If an electric current be now made to descend from A to B through such wire, the needle will be deflected into the position N S; that is, its north (1.) pole will turn to the right as you stand before it; and on increasing the force of the electric current, the deflection of the needle might be so far increased as to cause it to place itself at right angles to the electric current, as shown by the dotted figure; but this may be prevented by the small studs cc, by which such extreme deflection is limited. If we now suppose the wire to be continued on, and bent into the shape of the letter U, the current still pa ing onwards, that is, down the limb A B and up B C (as indicated by the darts), then the portion of the current B C would also act upon the needle, and proportionately increase the deflection. If we now change the direction of the electric current, so as to send it down C and up A, then also will the direct on of the deviation of the magnetic needle be changed, and its i\orth pole, instead of pointing to the right, will point to the left. And if, by repeated convolutions of the wire, the electric current be made to pa many times around the magnetic needle, the deflection would be still further increased, so that, by the adoption of this latter expedient, or the use of a coil instead of a single wire, the effect of the electric current may be so multiplied as to produce a considerable deflection by a comparatively feeble current. This is, in fact, the principle of the galvanometric multiplier, and is the form of apparatus used in the electric telegraph, as now most commonly constructed. The construction of this coil is shown iu the annexed wood-cut It consists of a polished wooden or ivory frame, round which are bound some hundred feet of fine copper wire, covered with silk, which, being a nonconductor, prevents the lateral pa age or transfer of electricity from wire to wire, so that the current, entering at A, pa es through the whole length of the coil, and goes out at B. Two magnetic needles are fixed 4 X [s. 1426]